The cooling system in the engine should (in theory) be a sealed environment, the coolant level should be up to the radiator cap with little to no air.
The res bottle is there to stop air getting into the system. The way it is designed to work with a certain pressure rating cap is that when the system heats up, the expanding coolant gets released into the res bottle.
When the system cools down after you finish driving the coolant contracts and creates a lower pressure inside the cooling system, the coolant in the reservoir bottle flows back into the system keeping air out of it.
If you are forever refilling this you should be looking for signs of coolant leaking anywhere and everywhere (if using proper coolant then it should be easy to spot any sign of green in and around all the pipes and fittings under the engine bay, corrosion around hoses fitting to pipes, split hoses, lose hose clamps, weeping seals on the radiator etc etc ), water does evaporate slowly, it's normal to have to top up the res bottle occasionally but you shouldn't have to be doing it on a daily/weekly basis, if there are no puddles under the car or you can't see any signs of external leakage then it's a good chance there's something internal going on, 95% of the time it's to do with the intake manifold gaskets, less likely but could be worse and be a head gasket but it's much easier and quicker to do the intake gaskets first and then keep an eye on the level afterwards to make sure it's stopped.
The engine i used in the original reply has been doing this for 4 years, every week or so when i was driving it all the time i'd have to top it up with 0.5-1 litre of fluid, i don't care to much about this engine as i have a fully rebuild donk ready to drop in. Purely for the hell of it i wondered how long this damn thing would last before it finally gave up...... If it was my daily driver and i needed a reliable car i wouldn't have been chancing it this long...... but the damn thing just keeps going...... not looking forward to finally stripping it down when i rip it out, gonna be pretty stuffed by that stage